Amniocentesis is a procedure used to diagnose fetal defects in the early
second trimester of pregnancy. A sample of the amniotic fluid, which
surrounds a fetus in the womb, is collected through a pregnant
woman's abdomen using a needle and syringe. Tests performed on
fetal cells found in the sample can reveal the presence of many types of
genetic disorders, thus allowing doctors and prospective parents to make
important decisions about early treatment and intervention.
Since the mid-1970s, amniocentesis has been used routinely to test for
, by far the most common, nonhereditary, genetic birth defect, afflicting
about one in every 1,000 babies. By 1997, approximately 800 different
diagnostic tests were available, most of them for hereditary genetic
disorders such as
Tay-Sachs disease, sickle cell anemia, hemophilia, muscular dystrophy
Amniocentesis, often called amnio, is recommended for women who will be
older than 35 on their due-date. It is also recommended for women who have
already borne children with birth defects, or when either of the parents
history of a birth defect for which a diagnostic test is available.
Another reason for the procedure is to confirm indications of Down
syndrome and certain other defects which may have shown up previously
during routine maternal blood screening.
The risk of bearing a child with a nonhereditary genetic defect such as
Down syndrome is directly related to a woman's age—the older
the woman, the greater the risk. Thirty-five is the recommended age to
begin amnio testing because that is the age at which the risk of carrying
a fetus with such a defect roughly equals the risk of miscarriage caused
by the procedure–about one in 200. At age 25, the risk of giving
birth to a child with this type of defect is about one in 1,400; by age 45
it increases to about one in 20. Nearly half of all pregnant women over 35
in the United States undergo amniocentesis and many younger women also
decide to have the procedure. Notably, some 75% of all Down syndrome
infants born in the United States each year are to women younger than 35.
One of the most common reasons for performing amniocentesis is an abnormal
alpha-fetoprotein (AFP) test. Alpha-fetoprotein is a protein produced by
the fetus and present in the mother's blood. A simple blood
screening, usually conducted around the 15th week of pregnancy, can
determine the AFP levels in the mother's blood. Levels that are too
high or too low may signal possible fetal defects. Because this test has a
high false-positive rate, another test such as amnio is recommended
whenever the AFP levels fall outside the normal range.
Amniocentesis is generally performed during the 16th week of pregnancy,
with results usually available within three weeks. It is possible to
perform an amnio as early as the 11th week, but this is not usually
recommended because there appears to be an increased risk of miscarriage
when done at this time. The advantage of early amnio and speedy results
lies in the extra time for decision making if a problem is detected.
Potential treatment of the fetus can begin earlier. Important, also, is
the fact that elective abortions are safer and less controversial the
earlier they are performed.
As an invasive surgical procedure, amnio poses a real, although small,
risk to the health of a fetus. Parents must weigh the potential value of
the knowledge gained, or indeed the reassurance that all is well, against
the small risk of damaging what is in all probability a normal fetus. The
serious emotional and ethical dilemmas that adverse test results can bring
must also be considered. The decision to undergo amnio is always a matter
of personal choice.
The word amniocentesis literally means "puncture of the
amnion," the thin-walled sac of fluid in which a developing fetus
is suspended during pregnancy. During the sampling procedure, the
obstetrician inserts a very fine needle through the woman's abdomen
into the uterus and amniotic sac and withdraws approximately one ounce of
amniotic fluid for testing. The relatively painless procedure is performed
on an outpatient basis, sometimes using local anesthesia.
The physician uses ultrasound images to guide needle placement and collect
the sample, thereby minimizing the risk of fetal injury and the need for
insertions. Once the sample is collected, the woman can return home after
a brief observation period. She may be instructed to rest for the first 24
hours and to avoid heavy lifting for two days.
The sample of amniotic fluid is sent to a laboratory where fetal cells
contained in the fluid are isolated and grown in order to provide enough
genetic material for testing. This takes about seven to 14 days. The
material is then extracted and treated so that visual examination for
defects can be made. For some disorders, like Tay-Sachs, the simple
presence of a telltale chemical compound in the amniotic fluid is enough
to confirm a diagnosis. Depending on the specific tests ordered, and the
skill of the lab conducting them, all the results are available between
one and four weeks after the sample is taken.
Cost of the procedure depends on the doctor, the lab, and the tests
ordered. Most insurers provide coverage for women over 35, as a follow-up
to positive maternal blood screening results, and when genetic disorders
run in the family.
An alternative to amnio, now in general use, is chorionic villus sampling,
or CVS, which can be performed as early as the eighth week of pregnancy.
While this allows for the possibility of a first trimester abortion, if
warranted, CVS is apparently also riskier and is more expensive. The most
promising area of new research in prenatal testing involves expanding the
scope and accuracy of maternal blood screening as this poses no risk to
It is important for a woman to fully understand the procedure and to feel
confident in the obstetrician performing it. Evidence suggests that a
physician's experience with the procedure reduces the chance of
mishap. Almost all obstetricians are experienced in performing
amniocentesis. The patient should feel free to ask questions and seek
emotional support before, during and after the amnio is performed.
Necessary aftercare falls into two categories, physical and emotional.
During and immediately following the sampling procedure, a woman may
, a rapid heartbeat, and cramping. Once past these immediate hurdles, the
physician will send the woman home with instructions to rest and to report
any complications requiring immediate treatment, including:
To perform amniocentesis, a physician uses an ultrasound monitor to
visualize the fetus while inserting a syring to extract amnniotic
fluid for analysis.
(Illustration by GGS Information Services.)
Once the procedure has been safely completed, the anxiety of waiting for
the test results can prove to be the worst part of the process. A woman
should seek and receive emotional support from family and friends, as well
as from her obstetrician and family doctor. Professional counseling may
also prove necessary, particularly if a fetal defect is discovered.
Most of the risks and short-term side effects associated with
amniocentesis relate to the sampling procedure and have been discussed
above. A successful amnio
sampling results in no long-term side effects. Risks include:
Alpha fetoprotein (AFP)
—A substance produced by a fetus' liver that can be found
in the amniotic fluid and in the mother's blood. Abnormally high
levels of this substance suggests there may be defects in the fetal
neural tube, a structure that will include the brain and spinal cord
when completely developed. AFP may also be found at elevated levels in
the blood of adults with liver, testicular, and ovarian cancer.
—A genetic defect resulting in the partial to complete absence of
the brain and malformation of the brainstem.
Chorionic villus sampling
—A procedure used for prenatal diagnosis at 10–12 weeks
gestation. Under ultrasound guidance a needle is inserted either through
the mother's vagina or abdominal wall and a sample of the
chorionic membrane. These cells are then tested for chromosome
abnormalities or other genetic diseases.
—A microscopic thread-like structure found within each cell of
the human body and consisting of a complex of proteins and DNA. Humans
have 46 chromosomes arranged into 23 pairs. Chromosomes contain the
genetic information necessary to direct the development and functioning
of all cells and systems in the body. They pass on hereditary traits
from parents to child (like eye color) and determine whether the child
will be male or female.
—A chromosomal disorder caused by an extra copy or a
rearrangement of chromosome 21. Children with Down syndrome have varying
degrees of mental retardation and may have heart defects.
—Refers to genes, the basic units of biological heredity, which
are contained on the chromosomes.
—Something which is inherited, that is passed down from parents
to offspring. In biology and medicine, the word pertains to inherited
Maternal blood screening
—Screening that is normally done early in pregnancy to test for a
variety of conditions. Abnormal amounts of certain proteins in a
pregnant woman's blood raise the probability of fetal defects.
Amniocentesis is recommended if such a probability occurs.
—An inherited disease caused by a missing enzyme that is
prevalent among the Ashkenazi Jewish population of the United States.
Infants with the disease are unable to process a certain type of fat
which accumulates in nerve and brain cells, causing mental and physical
retardation, and, finally, death.
—A medical test in which sound waves are directed against
internal structures in the body. As sound waves bounce off the internal
structure, they create an image on a video screen. Ultrasonography is
often used to diagnose fetal abnormalities, gallstones, heart defects,
and tumors. Also called ultrasound imaging.
An overall "normal" result does not, however, guarantee that
the pregnancy will come to term, or that the fetus does not suffer from
some other defect. Laboratory tests are not 100% accurate at detecting
conditions, nor can every possible fetal condition be tested for.
Positive results on an amnio analysis indicate the presence of the fetal
defect being tested for, with an accuracy approaching 100%. Prospective
parents are then faced with emotionally and ethically difficult choices
regarding treatment options, the prospect of dealing with a severely
affected newborn, and the option of elective abortion. At this point, the
parents need expert medical advice and counseling.
There is a risk of miscarrage with this procedure.
If there is excess bleeding, a doctor should be contacted.
Hassold, Terry and Schwartz, Stuart. "Chromosome Disorders."
Harrison's Principles of Internal Medicine
, ed. Eugene Braunwald, et al. Philadelphia: McGraw-Hill, 2001.
Miesfeldt, Susan and Jameson, J. Larry. "Screening, Counseling, and
Prevention of Genetic Disorders." In
Harrison's Principles of Internal Medicine
, ed. Eugene Braunwald, et al. Philadelphia: McGraw-Hill, 2001.
Interpretation of Diagnostic Tests,
7th ed. hiladelphia, PA: Lippincott Williams & Wilkens, 2000.
American College of Obstetricians and Gynecologists. 409 12th St., S.W.,
P.O. Box 96920, Washington, DC 20090-6920.
National Institutes of Health.